The invention relates to a subscriber's connecting network comprising a symmetrical power amplifier which is formed by two pairs of complementary transistors, the bases and the emitters of each pair of transistors being interconnected and forming two input and two output terminals, respectively, of the symmetrical amplifier, whereas the collectors of the transistors of one type are coupled to a first terminal of a d.c. voltage source and the collector of the transistors of the other type are coupled to a second terminal of the d.c. voltage source, the output terminals being connected to the terminals of a subscriber's line and the input terminals being coupled to the terminals of the d.c. voltage source via two input impedances.
As known, a subscriber's connecting network is an arrangement for connecting a subscriber's line to a telephone exchange and, by means of this arrangement, it must be possible to apply different voltages to the subscriber's line under the control of the exchange. Examples of these voltages are the ringing voltage having a frequency of, for example, 25 or 50 Hz, a d.c. voltage of a given polarity for transmitting dialling information and speech signals and a metering signal having a frequency of, for example, 12 kHz. For signalling purposes the subscriber's circuit must also be capable of reversing the polarity of the d.c. voltage on the subscriber's line.
The realization of these different voltages on the subscriber's line entails practical problems which can generally not be solved in a simple and economic manner. The ringing voltage, which is a sinusoidal voltage to prevent disturbing harmonics from occurring on the subscriber's line is derived from a bulky and expensive transformer. It is known to include a winding of such a transformer in a wire of the line, in which case this winding must then have a low impedance for speech signals. Alternatively, it is known to connect the transformer winding for the duration of the ringing signal to the terminals of the subscriber's line via relay contacts.
In Applicant's non-prepublished Netherlands Patent Application No. 7,810,301 it has been proposed to perform the above-mentioned supply functions without the use of a transformer or a relay but exclusively with solid-state integrated components by using a connecting network comprising a symmetrical power amplifier, two output terminals which are connected to the subscriber's line. Each input terminal of the power amplifier is connected to the output of a digital-to-analogue converter, an input terminal of each digital-to-analogue converter being connected to one or the other terminal of the d.c. voltage source via a switching network. The switching networks are controlled in anti-phase by a binary digital signal generated under the control of the telephone exchange, the digital signal characterizing the voltage desired for the subscriber's line.
Feeding the subscriber's line via a symmetrical power amplifier gives rise to problems in limiting the current in the transistors of this amplifier in the case of very short lines or accidental short-circuiting of the line. Further, the different administrations require the current derived from the supply source to be limited in the subscriber's line by a resistor having a predetermined value R, for example 300.OMEGA. in France.
In order to protect the transistors of the symmetrical amplifier and at the same time satisfy the requirements of the administrations, this current limitation can be realized by arranging two resistors, each having a value R/2, between the two output terminals of the symmetrical amplifier and the two ends of the subscriber's line. This, however, has the drawback that a by-pass capacitor for the speech current must be connected to the terminals of each of these resistors. Each capacitor must have a comparatively large capacitance and must be capable of carrying a d.c. voltage having a value of half the supply voltage into two directions. In order to guarantee the symmetry of the amplifier, the two capacitors must be as equal as possible. This current limitation method therefore requires comparatively expensive and bulky by-pass capacitors.
Current limitation by means of the polarized transistors of the symmetrical amplifier, the transistors playing the role of resistance, results in such a high dissipation in these transistors that integration is very difficult.
It is an object of the invention to provide a subscriber's connecting network of the type mentioned in the preamble, the current in the subscriber's line and in the transistors of the symmetrical amplifier being limited so as to enable the integration of as many components as possible.
According to the invention the subscriber's connecting network is therefore characterized in that in the collector circuit of the transistors of one type and in the collector circuit of the transistors of the other type two equal resistors are included for limiting the current in the subscriber's line. Means are provided for rendering the base voltages of the transistors dependent on the collector voltages thereof and a by-pass capacitor is arranged between the input terminals of the symmetrical power amplifier for the speech currents in the subscriber's line.
The result of such an arrangement is that the transistors of the symmetrical amplifier can operate near saturation so that the dissipated power is sufficiently low to enable integration. In addition, the by-pass capacitor may have a rather low value, for example a hundred times lower than would be required for a capacitor applied across the terminals of a current limiting resistor.